The purpose of this research is to determine the central control mechanisms for speech and voice functioning. Central activation during speech perception and production is studied as well as responses of the vocal tract musculature to transcranial magnetic stimulation. Brain stem reflexes in response to neurophysiological stimuli are studied to examine the role of sensory-motor feedback on the speech and voice musculature in both normal and disordered speech and voice production. 1. With the addition of more normal volunteers and patients, the positron emission tomographic study of brain activation during a temporal order speech perception task was completed. No specific brain region was activated by the speech perception task in the normal subjects once the data were corrected for overall differences in metabolic levels between the two scans. In the patients, hypometabolism was identified only on the activated scan in the patients with left-sided foci. 2. A preliminary study employing transcranial magnetic stimulation demonstrated bilateral laryngeal muscle responses to stimulation on either the right or left sides. However, stimulation on the left side produced increased amplitudes and reduced response latencies on both sides of the larynx in comparison with stimulation on the right. Similarly, a lower level of magnetic stimulation was needed on the left than on the right to produce a response in the laryngeal muscles during threshold testing. 3. A consistent period of suppression between 25 and 50 ms followed by a polyphasic response between 55 and 70 ms was found during electrical stimulation of the recurrent laryngeal nerve. These long latency responses increased when muscle activation levels were higher and greater stimulation currents were used.